It is of frequent occurrence in the course of hydrometallurgy operations to have to separate mixtures of base metals such as magnesium, cobalt, nickel, zinc, and copper from manganese and iron, in the presence of variable amounts of arsenic and antimony. These complex solutions may result from acid digestion of polymetallic ores, either with hydrochloric acid or sulfuric acid, at atmospheric pressure or in autoclaves. Or they might originate from sulfide ores previously oxidized before acid leaching.
There is a number of methods to isolate metals or groups of metals from such polymetallic solutions. Selective precipitations, using bases such as sodium hydroxide NaOH or calcium hydroxide Ca(OH)2, or even the corresponding carbonates (Na2CO3, CaCO3), have been reported in many instances. More recently, ion exchange resins have been developed with rather specific capabilities towards a given cation or group of cations. As an alternate method, solvent extraction with a complexing agent is now of frequent use. In most cases, however, a pH adjustment is needed for these specific separation methods with solvent or resins, and the presence of iron often creates interferences.
In available methods for selectively extracting the base metals from the polymetallic solution, namely, solvent extraction, ion exchange resins, precipitation by pH adjustment, electrowinning for example, the pH is a key factor for a selective extraction. In many instances, it was found that, particularly with solutions rich in iron, this pH adjustment represented a very delicate step, since the precipitation of iron without losses of nickel, cobalt or copper called for a very fine control over said pH, at the level of 0.1 pH unit. Such a control, although possible at a small scale, becomes nearly impossible in a large reactor due to inhomogeneity and mixing problems. Also, the iron is often present at its two common valencies, ferrous and ferric, and these two species have vastly different solubilities at various pH. The net result is that ferric ion precipitate, while ferrous material is still soluble and readily reoxiding after filtration, leading to uncontrollable situations.
Therefore, it has been found desirable to develop a method for selectively treating polymetallic solutions containing a multiplicity of base metals associated with iron, for recovery of base metals of value without overlapping precipitation of iron-bearing fractions. This is particularly true if the acid leachate contains arsenic and antimony, the removal of these two metals from the solution being required for environmental reasons.